Digital radio terminals have become particularly advantageous in a number of key types of communication. High frequency point to point communications are used by, among others, cellular operators, telecommunications operators, private network operators, governments, and large telecommunications operations.
While many modulation techniques are available for use on microwave digital radios, such as QPSK, QAM and so on, cost and other issues have militated in favor of the use of direct modulated oscillators, or direct modulated FSK systems.
Operators of such microwave digital radios are typically assigned to specific frequencies, or channels, for their communications. Each channel is characterized by a center frequency and a spectrum emission mask or template which permits a higher energy level at the center frequency and decreasing energy levels as the transmitted frequency diverges (in either direction) from the center frequency. The spectrum emission mask, sometimes referred to simply as the "mask," is defined by the federal government, and transmissions outside the mask can interfere with transmissions on adjacent channels as well as resulting in serious adverse consequences to the system operator. Such interference with neighboring channels is referred to "stepping on" those channels.
Superficially, it would seem to be straightforward to avoid stepping on adjacent channels simply by setting the direct modulated FSK occupied bandwidth of the system. However, this has been proven not to be correct. Most importantly, it is now recognized that the transmit occupied bandwidth--and therefore radio performance--can vary significantly with temperature or frequency in a direct modulated FM system. While temperature varies relatively slowly, it can vary over a significant range. This can cause a carefully tuned output spectrum to exceed the mask.
The historical approach to compensate for temperature variations and avoid stepping on adjacent channels has been to reduce the maximum bandwidth; however, this has the unacceptable effect of reducing the FM demodulated signal-to-noise ratio. This reduction in the demodulated signal amplitude can result in significantly poorer performance for the radio network. A common approach to representing such degradation is to perform a conversion of the occupied bandwidth from the frequency domain to the time domain. Where multiple digital modulation levels are used, the result of the time conversion is a plurality of random time-variant waveforms of different levels which are generally arcuate and, plotted together, take the general shape of an eye. This is frequently referred to as "the eye", and such terminology will be used from time to time hereinafter. A reduction in the demodulated signal amplitude--and the corresponding reduction in the occupied bandwidth--basically is depicted in the eye by the arcuate waveforms which form the eye becoming less arcuate (i.e., flatter) and moving closer to one another, such that the overall impression is that the eye opening becomes smaller. An enhancement in the demodulated signal amplitude--and the corresponding increase in occupied bandwidth--is depicted by the waveforms becoming more arcuate and moving further apart. This is commonly referred to as the eye becoming larger. A larger eye is generally more desirable.
Variations in frequency, even with constant temperature, can also lead to significant variation in occupied bandwidth. Thus, for tunable systems which can be operated at any of a wide range of frequencies, undesirable occupied bandwidth changes can result from changes in selected channel. For many operators of microwave radio systems, the frequency of operation is chosen on-site. Thus, the occupied bandwidth of the system must be readily configurable outside of the manufacturing facility, and must take into account the variations in occupied bandwidth which can result from changes in frequency at even a stable temperature.
As a result, there has been a long-felt need for a system which dynamically maintains optimized occupied bandwidth over a significant range of operating temperatures and frequencies.